Nitrocarburized crankshaft member and steel for nitrocarburized crankshafts

ABSTRACT

A nitrocarburized crankshaft member made of a steel having essentially ferrite and perlite, and at least a portion of a steel surface thereof having a ferrite surface area of 50% or greater that is imparted with a nitrocarburized hard layer. The steel consists of C, Si, Mn, Cu, Ni, and Cr as required elements and Mo, N, s-Al, Ti, Pb, Bi, and Ca as optional elements that may be included, and Fe and inevitable impurities. C is within a range of 0.25 to 0.32%. The nitrocarburized crankshaft member includes a thickness of a surface compound layer of the nitrocarburized hard layer of 10 to 35 μm that is formed during establishment of a diffusion depth of a nitrogen diffusion zone below the surface compound layer of 700 μm or greater.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation Application that claimspriority under 35 USC 120 to U.S. patent application Ser. No. 12/958,855filed on Dec. 2, 2010, and U.S. Provisional Patent Application No.61/353,018 filed on Jun. 9, 2010, and claims priority under 35 U.S.C.§119 to Japanese Application No. 2009-275908 filed on Dec. 3, 2009, theentire contents of each of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitrocarburized crankshaft memberwherein at least a portion of the surface is nitrocarburized by anitrocarburizing process, and a steel for nitrocarburized crankshaftsthat can be used therefor.

2. Description of the Background Art

In a reciprocating engine of an automobile or the like, a crankshaft isused to extract an output of rotating motion from a piston that linearlyreciprocates. This crankshaft, as shown in the drawings ofJP2007-197812A, for example, comprises a journal portion located arounda shaft that is the same as the rotating shaft of an output shaft, a pinportion located around a shaft that moves the rotating shaft in parallelby a predetermined distance, and an arm portion provided in a pluralityat a predetermined interval along the rotating shaft, connecting thejournal portion and pin portion.

In the manufacturing method of this bent shape crankshaft, for example,a steel rod is hot-forged into the required shape, and this roughlyformed shape is then machined into an integrated crankshaft member.Subsequently, as required, a normalizing process is carried out toremove residual stress, and a surface hardening process such asnitrocarburizing or nitriding is carried out to improve fatiguestrength. During the forging or the surface hardening process, thecrankshaft member is likely to bend or warp. Therefore, subsequently, abending correction is carried out to correct bending, warping, and thelike.

Any bends or warps of the crankshaft member is corrected by a bendingcorrection bending the crankshaft member in the direction opposite thedirection of the bend. At this time, cracks readily occur on the hardsurface layer formed by a surface hardening process, such asnitrocarburizing or nitriding, and these cracks may cause damage to thecrankshaft. Given this factor, a crankshaft member made of a steel thatis superior in bending correctability, which makes it possible toperform a bending correction process without imparting excessive strain,has been in demand.

In response to such a demand, there have been developed steels forcrankshafts that are made of a medium-carbon steel wherein the carboncontent is suppressed to about 0.3 wt % to achieve a reduction inaverage hardness. For example, JP2002-226939A discloses a steel fornitrocarburized crankshafts that includes C in the amount of 0.2 to 0.6wt %, Si in the amount of 0.05 to 1.0 wt %, Mn in the amount of 0.25 to1.0 wt %, S in the amount of 0.03 to 0.2 wt %, Cr in the amount of 0.2wt % or less, s-Al in the amount of 0.045 wt % or less, Ti in the amountof 0.002 to 0.010 wt %, N in the amount of 0.005 to 0.025 wt %, and O inthe amount of 0.001 to 0.005 wt %, and satisfies the condition 0.12×Tiwt %<O wt %<2.5×Ti wt % and 0.04×N wt %<O wt %<0.7×N wt %.

Now, from the viewpoints of cost and ease of manufacture, a solutionstrategy for the demand described above that proactively uses Cu, whichmay exist as a trapped element in scraps of raw material, is highlypreferred. For example, the technical journal “DENKI-SEIKOU”, Volume 77,No. 1 (February 2006), discloses that adding Cu to medium-carbon steelcan increase ferrite hardness, improving yield strength, and form aharder yet thinner compound layer, which is formed by nitrocarburizingThat is, by adding Cu to a medium-carbon steel, the achievement of anitrocarburized crankshaft member that is superior in bendingcorrectability and superior in fatigue strength is expected.

Steels for nitrocarburized crankshafts made of a medium-carbon steelthat include Cu are disclosed in JP2002-226939A, for example. This steelfor nitrocarburized crankshafts includes C in an amount greater than orequal to 0.30 wt % and less than or equal to 0.50 wt %, Si in an amountgreater than or equal to 0.05 wt % and less than or equal to 0.30 wt %,Mn in an amount greater than or equal to 0.50 wt % and less than orequal to 1.00 wt %, S in an amount greater than or equal to 0.03 wt %and less than or equal to 0.20 wt %, Cu in an amount greater than orequal to 0.05 wt % and less than or equal to 0.60 wt %, Ni in an amountgreater than or equal to 0.02 wt % and less than or equal to 1.00 wt %,and Cr in an amount greater than or equal to 0.05 wt % and less than orequal to 0.30 wt %, and satisfies the condition that compositionparameter F1>20 and F2>0 given F1=185 W_(Cr)+50 W_(Cu), and F2=8+4W_(Ni)+1.5 W_(Cu)−44 W_(Cr), where W_(Cu), W_(Ni), and W_(Cr) are thecontent percentages (unit: wt %) of Cu, Ni, and Cr, respectively.

It is now expected that a crankshaft member that increases theperformance requirements of crankshafts, excels in cost and ease ofmanufacture, and achieves both high fatigue strength and superiorbending correctability will be developed.

The present invention was made in view of such circumstances, and it istherefore an object of the present invention to obtain a nitrocarburizedcrankshaft member having at least a portion of its surface subjected tonitrocarburizing, excels in cost and ease of manufacture, and achievesboth high fatigue strength and superior bending correctability comparedto prior art crankshaft members, and a steel for nitrocarburizedcrankshafts that can be used therefor.

SUMMARY OF THE INVENTION

The inventors discovered that a reduction in the amount of carbon inmedium-carbon steel increases the bending correctability of anitrocarburized crankshaft member and, with the addition of apredetermined amount of Cu in response to the reduction in yieldstrength caused by the decrease in the amount of carbon, achieves acrankshaft steel that is also superior in fatigue strength. Theinventors further discovered that regulating the amount of added Cu towithin a predetermined range in response to Ni, which significantlyaffects the surface-hardened layer produced by nitrocarburizing,suppresses the growth of a compound layer formed on the outermostsurface of the surface-hardened layer. Here, the inventors conducted alarge number of tests to assess the effects of other elements on theadded amount of Cu, interpolating the effects between the elements ofeach test by multiple regression calculations, and finalized theinvention. That is, according to the crankshaft steel of the presentinvention, a crankshaft member provided with a special surface-hardenedlayer produced by known nitrocarburizing can be achieved.

That is, the nitrocarburized crankshaft member according to the presentinvention is a member made of a steel having essentially ferrite andperlite, wherein the steel includes C in an amount by weight of 0.25 to0.32% as a required element and an optional element that may beincluded, and Fe and inevitable impurities in the remaining portion, andat least a portion of the steel surface having a ferrite surface area of50% or greater is imparted with a nitrocarburized hard layer, wherein:the steel includes C, Si, Mn, Cu, Ni, and Cr as the required elements,and Mo, N, s-Al, and Ti as the optional elements, in the amounts byweight of Si within the range of 0.01 to 0.15%, Mn within the range of0.55 to 0.90%, Cu within the range of 0.10 to 0.60%, Ni within the rangeof 0.05 to 0.30%, Cr within the range of 0.10 to 0.20%, Mo within therange of 0.05% or less, N within the range of 0.020% or less, s-Alwithin the range of 0.020% or less, and Ti within the range of 0.020% orless, and satisfies the conditions that 0.43≦C_(eq)≦0.53 whereC_(eq)=C+0.07×Si+0.16×Mn+0.19×Cu+0.17×Ni+0.2×Cr+0.4×Mo, and0.38≦C_(req)≦0.47 whereC_(req)=0.55×C+0.06×Si+0.18×Mn+0.09×Cu+0.47×Mo+1.0×Cr; the Ni to Cuweight ratio is 0.50≦Ni/Cu≦1.00; and the amounts by weight of P and S,which are inevitable impurities, are suppressed to 0.03% or less and0.15% or less, respectively, thereby suppressing the thickness of asurface compound layer of the nitrocarburized hard layer to 10 to 35 μmwhile establishing a diffusion depth of a nitrogen diffusion zone belowthe surface compound layer of 700 μm or greater.

According to the invention, compared to prior art crankshaft members,both high fatigue strength and superior bending correctability can beachieved. That is, with a relatively low amount of carbon and theaddition of a predetermined amount of Cu, it is possible to achieve aspecial nitrocarburized hard layer that increases yield strength andenhances fatigue strength while suppressing the thickness of the surfacecompound layer of the nitrocarburized hard layer to 10 to 35 μm andextending the diffusion depth of the nitrogen diffusion zone below thesurface compound layer to 700 μm or greater. This specialnitrocarburized hard layer can be achieved by adding at least therequired elements C, Si, Mn, Cu, Ni, and Cr to the steel inpredetermined amounts. In particular, since Cu can be added in an amountof 0.10 wt % or greater, it is possible to utilize trapped elements inscrap and further achieve a crankshaft that is superior in cost and easeof manufacture.

In the above-described invention, the HV ratio of the maximum hardnessof the nitrogen diffusion zone, at a depth of at least 50 μm or greaterfrom the surface compound layer, to the core hardness may be 1.65 orgreater. According to the invention, both a higher fatigue strength andsuperior bending correctability of the crankshaft member can beachieved.

Further, in the above-described invention, the core hardness may bewithin the range of HV 160 to 190. Further, the maximum hardness of thenitrogen diffusion zone may be within the range of HV 280 to 330. Insuch a nitrocarburized crankshaft member provided with a specialnitrocarburized hard layer as described above, both a higher fatiguestrength and superior bending capability can be achieved.

Furthermore, the steel for nitrocarburized crankshafts according to thepresent invention is a steel having essentially ferrite and perlite,wherein the steel includes C in an amount by weight of 0.25 to 0.32% asa required element and an optional element that may be included, and Feand inevitable impurities in the remaining portion, and has a ferritesurface area of 50% or greater, wherein: the steel includes C, Si, Mn,Cu, Ni, and Cr as the required elements and Mo, N, s-Al, and Ti as theoptional elements, in the amounts by weight of Si within the range of0.01 to 0.15%, Mn within the range of 0.55 to 0.90%, Cu within the rangeof 0.10 to 0.60%, Ni within the range of 0.05 to 0.30%, Cr within therange of 0.10 to 0.20%, Mo within the range of 0.05% or less, N withinthe range of 0.020% or less, s-Al within the range of 0.020% or less,and Ti within the range of 0.020% or less, and satisfies the conditionsthat 0.43≦C_(eq)≦0.53 whereC_(eq)=C+0.07×Si+0.16×Mn+0.19×Cu+0.17×Ni+0.2×Cr+0.4×Mo, and0.38≦C_(req)≦0.47 whereC_(req)=0.55×C+0.06×Si+0.18×Mn+0.09×Cu+0.47×Mo+1.0×Cr; the Ni to Cuweight ratio is 0.50≦Ni/Cu≦1.00; and the amounts by weight of P and S,which are inevitable impurities, are suppressed to 0.03% or less and0.15% or less, respectively.

According to the invention, compared to prior art crankshaft members,both high fatigue strength and superior bending correctability can beachieved. That is, with a relatively low amount of carbon and theaddition of a predetermined amount of Cu, it is possible to make such asteel with increased yield strength and enhanced fatigue strength whilesuppressing the thickness of the surface compound of the nitrocarburizedhard layer to a predetermined value or less and extending the diffusiondepth of the nitrogen diffusion zone under the surface compound layer bya predetermined amount or more. This special nitrocarburized hard layercan be achieved by adding at least the required elements C, Si, Mn, Cu,Ni, and Cr to the steel in predetermined amounts. In particular, sinceCu can be added in an amount of 0.10 wt % or greater, it is possible toutilize trapped elements in scrap and further achieve a crankshaft thatis superior in cost and ease of manufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is table of element compositions of steel for crankshafts of thepresent invention and comparative examples;

FIG. 2 is a flowchart of the manufacturing process of the crankshaft;

FIG. 3 is a plane view of the crankshaft;

FIG. 4 is a diagram illustrating a cross section of the nitrocarburizedhard layer of the crankshaft member according to the invention; and

FIG. 5 is a table of the various test results of the steel forcrankshafts of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following describes in detail several embodiments of the crankshaftmember and steel for crankshafts therefor according to the presentinvention.

First, the manufacturing method of the crankshaft member will bedescribed with reference to FIG. 2.

As indicated by the element compositions of FIG. 1, the steel forcrankshafts of the embodiments is a medium-carbon steel that includescarbon in the relatively small amount of 0.25 to 0.32 wt %. Onedistinguishing characteristic of this steel is that the steel caninclude Cu which can exist as a trapped element in general raw materialscrap as a required element.

The steel for crankshafts of the embodiments that consists essentiallyof ferrite and perlite is achieved by subjecting the steel of theelement compositions of FIG. 1 to a forming process (S1), such as hotforging, as necessary in order to bring the steel closer to the shape ofthe final product (refer to FIG. 3) and then implementing heat treatment(S2), which includes controlled cooling.

This steel for crankshafts is then subjected to machining (S3) asnecessary and subsequently a nitrocarburizing process (S4), whichimparts a nitrocarburized hard layer on at least a portion of thesurface, thereby obtaining the nitrocarburized crankshaft member of theembodiments.

As shown in FIG. 3, a crankshaft 1, which is the final product,comprises for example a journal portion 2 located around a shaft that isthe same as a rotating shaft X of the output shaft, a pin portion 3located around a shaft at a predetermined distance away from therotating shaft X, and an arm portion 4 that is provided in a pluralityat a predetermined interval along the rotating axis X, connecting thejournal portion 2 and the pin portion 3.

The crankshaft 1 is subjected to nitrocarburizing on the connectionportion of the journal portion 2 and the arm portion 4 where stressreadily concentrates due to the bent shape thereof, and on base Rportion 6 of the connection portion of the pin portion 3 and arm portion4. In this nitrocarburizing process, nitrogen is diffused from thesurface in order to increase the fatigue strength of the overallcrankshaft 1.

After the nitrocarburizing process (S4), the nitrocarburized crankshaftmember is subjected to a bending correction process (S5) as necessary.The bending correction process imparts bending strain in the directionopposite the bending direction.

Next, the nitrocarburizing process (S4) will be described in detail. Asillustrated in FIG. 4A, when nitrocarburizing is performed from thesurface in the crankshaft steel having ferrite+perlite, a surfacecompound layer 11 comprising compounds including a nitride is formed onthe outermost surface. Further, a nitrogen diffusion zone 12 of diffusednitrogen is formed on the inner side thereof. A core portion 13 keeps astructure, ferrite and perlite, of the crankshaft steel as is.

As illustrated in FIG. 4B, the surface compound layer 11 is extremelyhard since it is made of compounds. The underlying nitrogen diffusionzone 12 is a zone in which the nitrogen which had passed through thesurface compound layer 11 from the surface is diffused toward the innerportion, and thus comprises a hardness that is hardest on the sidenearest the surface compound layer 11 and gradually decreases to thesame hardness as the core portion 13. Note that the surface compoundlayer 11 and the nitrogen diffusion zone 12, which are slightly harderthan the core portion 13, are also referred to as a surface-hardenedlayer 10.

To further increase the fatigue strength of the final crankshaft 1, itis preferable to increase the maximum hardness as well as the diffusiondepth of the nitrogen diffusion zone 12. On the other hand, the surfacecompound layer 11 is hard and brittle and, thus to ensure that cracks donot occur, it is generally preferable to decrease the thickness whilemaintaining a constant hardness. The following describes how thesurface-hardened layer 10 is formed by the nitrocarburizing process (S4)when improving the fatigue strength of the crankshaft 1.

As illustrated in FIG. 4B, the maximum hardness of the nitrogendiffusion zone 12 (the hardness directly underneath the compound layer11) can be increased along with the diffusion depth by lengthening theexecution time of nitrocarburizing (FIG. 2, S4). That is, a curve 21 ofFIG. 4B can be changed to a curve 23. However, the thickness of thesurface compound layer 11 also unfortunately increases.

Therefore, an experiment was conducted by performing nitrocarburizing ona steel that includes carbon in the amount of 0.25 to 0.32 wt %, whichis a reduced amount of the carbon of medium-carbon steel, and containsadded Cu to increase fatigue strength. The details thereof will now bedescribed.

Each steel comprising the components shown in FIG. 1 was melted andsteel-made in a high-frequency induction furnace to obtain an ingot(while the studied steel is a steel that includes C in the amount of0.25 to 0.32 wt % with an added amount of Cu as described above. Thefigure also shows comparative examples 11 and 15 that are made of asteel that contains C in an amount greater than 0.32 wt %, andcomparative example 14 that is made of a steel that includes C in anamount less than 0.25 wt %). Each ingot was coarsely forged to a sizehaving a cross-section of 70 square millimeters, heated for a period of90 minutes at 1200 degrees Celsius, and then hot forged to a size havinga cross-section of 40 square millimeters. With an ending temperature ofat least 950 degree Celsius or higher, the forged square rod was thencontrol-cooled, particularly the rod being cooled from 700 to 600degrees Celsius at a cooling rate of approximately 0.2 to 0.5 degreesCelsius per second. The rod was then subjected to a gas nitrocarburizingfor 2 hours at 600 degrees Celsius (gas flow ratio NH₃:N₂:CO₂=53:42:5),and then quenched in an 80-degree Celsius oil bath to obtain each testpiece.

Next, the effects of the nitrocarburizing process on the test pieceswere measured. Each test piece was sliced, polished, and etched using apicral. The test pieces were then measured for the following: thickness(Dc) and hardness (Hc) of the surface compound layer 11, maximumhardness (hardness Hd at a depth of 50 μm from the surface compoundlayer 11) and diffusion depth (Dd) of the nitrogen diffusion zone 12,and core hardness (hardness Hb at a depth of 2 mm from the surfacecompound layer 11). Hardness was measured using a Vickers hardnesstester. Additionally, the percentage of ferrite in the cross-section wasfound from an optical image by image analysis, and the ferrite areapercentage was measured.

As a result of the above, a number of steels were found to still have athin surface compound layer 11 yet a higher hardness and greaterdiffusion depth in the nitrogen diffusion zone 12 than prior art steel.For example, given an ideal specification L of the surface-hardenedlayer 10 of a crankshaft member, the following was found to beachievable:

-   a hardness (Dc) of the surface compound layer 11 of 10 to 35 μm or    less,-   a maximum hardness (Hd) of the nitrogen diffusion zone 12 of HV 280    to 330,-   a diffusion depth (Dd) of the nitrogen diffusion zone 12 of 700 μm    or greater, and-   a hardness (Hb) of the core portion 13 of HV 160 to 190.

As shown in FIG. 5, all steels other than comparative examples 14 and 18satisfy the specification L for the diffusion depth (Dd) of the nitrogendiffusion zone 12. While the embodiments satisfy the specification L forthe thickness (Dc) of the surface compound layer 11, comparative example17 does not. Further, only comparative example 14 does not satisfy thespecification L for the hardness (Hb) of the core portion 13.Furthermore, the steels of comparative examples 12, 13, 15, 16, and 18do not satisfy the specification L for maximum hardness (Hd) of thenitrogen diffusion zone.

Now, although Cu was added as described above, a Cu loss during hotforging was found to significantly impact the yield strength of thesteel for crankshafts. In response, the addition of Ni by a specificquantity was tested. According to the tests, when Ni is included at a Nito Cu weight ratio of

0.50≦Ni/Cu≦1.00,

observations found that the Cu loss during hot forging can besuppressed.

Now, in the steels of the embodiments that satisfy the above-describedspecification L, the contributions of C, Si, Mn, Cu, Cr, and Mo, whichare elements that can significantly impact the maximum hardness (Hd) ofthe nitrogen diffusion zone 12, are summarized in the equation below asthe value Creq by multiple regression calculation. That is:

C_(req) value=0.55×C+0.06×Si+0.18×Mn+0.09×Cu+0.47×Mo+1.0×Cr   (Equation1)

According to this equation, when the range of the C_(req) value is 0.380to 0.470, a steel for crankshafts of the group with the embodiments ofFIG. 1 can be achieved. That is, particularly according to thenitrocarburizing process described above, a crankshaft member thatsatisfies the range of maximum hardness (Hd) of the nitrogen diffusionzone 12 of the specification L can be achieved.

On the other hand, the steels in comparative examples 12, 13, 15, 16,and 18 are out of the range of the maximum hardness (Hd) of the nitrogendiffusion zone 12 of the specification L, particularly as a result ofthe above-described nitrocarburizing. In these steels, the C_(req) valueis not within the range of 0.380 to 0.470. Conversely, aside fromcomparative example 14 which has a low amount of C, the steels incomparative examples 11 and 17 are within the range of theabove-described maximum hardness (Hd) and have a C_(req) value in therange of 0.380 to 0.470. This also confirms the relationship between therange of the C_(req) value and the maximum hardness (Hd) of the nitrogendiffusion zone 12.

Next, the relationship between the fatigue strength of the crankshaft 1and the yield strength of the steel for crankshafts will be described.

To further increase the fatigue strength of the final crankshaft 1, theyield strength of not only the nitrogen diffusion zone 12 but also thesteel for crankshafts is preferably increased. However, increasing yieldstrength generally makes the bending correction process (S5) difficultto achieve. Hence, the hardness associated with yield strength needs tobe at a certain level. That is, the hardness (Hb) of the above-describedcore portion 13 needs to be about 160 to 190 HV. With theabove-described specification L, the hardness ratio of the hardness (Hb)of the core portion 13 to the maximum hardness (Hd) of the nitrogendiffusion zone 12 determines the hardness gradient within the nitrogendiffusion zone 12. The maximum hardness (Hd) is about 280 to 330 HV,with a hardness ratio (HV ratio) of 1.65 or greater.

Similar to the maximum hardness (Hd) of the nitrogen diffusion zone 12by nitrocarburizing, the contributions of C, Si, Mn, Cu, Ni, Cr, and Mo,which are elements that can significantly impact the core hardness (Hb),are summarized in the equation below as the value C_(eq) by multipleregression calculation. That is:

C_(eq) value=C+0.07×Si+0.16×Mn+0.19×Cu+0.17×Ni+0.2×Cr+0.4×Mo   (Equation2)

According to this equation, when the Ceq value range is 0.430 to 0.530,a crankshaft member (crankshaft steel) that satisfies theabove-described core hardness (Hb) can be achieved given a steel forcrankshafts of the group with the embodiments of FIG. 1.

As shown in FIG. 5, excluding comparative examples 15 and 16, each ofthe crankshaft steels of the embodiments has a ferrite area percentageof 50% or higher. Note that when the ferrite area percentage exceeds80%, the core hardness (Hb) lowers significantly, resulting ininsufficient hardness of the crankshaft member as a whole.

From the above, it is confirmed that both high fatigue strength andsuperior bending correctability compared to prior art crankshaft memberscan be achieved when the crankshaft 1 is manufactured by executing thenitrocarburizing process as described above using a steel forcrankshafts having an element composition shown in embodiments 1 to 8 ofFIG. 1, in accordance with the process shown in FIG. 2.

The guidelines for obtaining the range of each component in the studiesof the steels of the embodiments are as described below.

C, in the range of general medium-carbon steel, increases yield strengthand enhances fatigue strength but does not achieve the bendingcorrectability required by the nitrocarburized crankshaft member. Therange, therefore, is 0.25 to 0.32 wt %.

Si improves fatigue strength and functions as a deoxidizing agent duringsteel welding. On the other hand, when Si is added in excess, a decreasein bending correctability results. Therefore, to ensure that Si does notaffect the nitrocarburizing process, the amount is within the range of0.01 to 0.15 wt %.

Mn, in general medium-carbon steel, improves the yield strength andenhances fatigue strength when added in a suitable range, and producesan Mn sulfide when combined with S, making it possible to improvemachinability. The range, therefore, is 0.55 to 0.90 wt %.

S, in general medium-carbon steel, improves machinability, but candecrease toughness when added in excess. The range, therefore, is 0.040to 0.150 wt %, preferably 0.040 to 0.070 wt %.

Cu improves the yield strength of steel as described above, achieves athin compound layer in nitrocarburizing, and enhances fatigue strengthwhen added in a suitable range. Further, from the viewpoint of cost andease of manufacture, which are objects of the present invention, theamount of Cu as elements trapped in general scrap needs to be a certainamount or more. On the other hand, when Cu is added in excess, adecrease in hot workability results. The range, therefore, is 0.10 to0.60 wt %, preferably 0.10 to 0.30 wt %.

Ni improves the ductility of perlite in the nitrogen diffusion zone,achieves a thin compound layer in nitrocarburizing, and improves bendingcorrectability when added in a suitable range. On the other hand, whenNi is added excessively, the Ni decreases machinability and competeswith the Cu in the decrease of compound layer thickness. The range,therefore, is 0.05 to 0.30 wt %.

Cr, in general medium-carbon steel, increases the strength and toughnessof the steel, making it possible to improve fatigue strength, when addedin a suitable range. The range, therefore, is 0.10 to 0.20 wt %.

Mo, in general medium-carbon steel, maintains hardness afternitrocarburizing and forging, increases the strength of the steel, andimproves fatigue strength when added in a suitable amount. On the otherhand, when Mo is added in excess, machinability decreases. The range,therefore, is 0.05 wt % or less.

Note that elements such as N, s-Al and Ti, can be included based on thepurpose of addition, within a range that does not impact the effectsachieved by the above-described required elements. For example, N, s-Aland Ti may be included in the amounts of 0.020 wt % or less, 0.020 wt %or less, and 0.020 wt % or less, respectively.

Furthermore, inevitable impurities that can be unavoidably includedduring manufacturing are preferably restricted as follows. That is, Pmay be included in the amount of 0.030 wt % or less.

Thus the present inventors discovered that reducing the amount of carbonin medium-carbon steel increases the bending correctability of anitrocarburized crankshaft member and, by adding a predetermined amountof Cu in response to the decrease in yield strength caused by thedecrease in the amount of carbon, achieves a crankshaft steel superiorin fatigue strength. On the other hand, the growth of the compound layerformed on the outermost surface of the surface-hardened layer issuppressed by regulating the amount of Cu to within a predeterminedrange with respect to Ni, which has a significant effect on thesurface-hardened layer achieved by nitrocarburizing. That is, regulatingthe amount of Cu achieves both a deeper nitrogen diffusion zone when apredetermined compound layer thickness similar to that of prior art isobtained, and a thinner compound layer when a nitrogen diffusion zonesimilar to that of prior art is obtained. As a result, both high fatiguestrength and superior bending correctability of the crankshaft membercan be achieved.

While the above has described representative embodiments of the presentinvention and modifications based thereon, the present invention is notlimited thereto and suitable modifications can be made by those skilledin the art. It will be apparent to those skilled in the art that variousmodifications can be made in the present invention without departingfrom the spirit or scope of the present invention. For example, tofurther improve machinability, Pb, Bi, and Ca, which are elements knownto have such an effect, may be added as well.

1. A nitrocarburized crankshaft member made of a steel havingessentially ferrite and perlite, and at least a portion of a steelsurface thereof having a ferrite surface area of 50% or greater that isimparted with a nitrocarburized hard layer, wherein the steel consistsof C, Si, Mn, Cu, Ni, and Cr as required elements and Mo, N, s-Al, Ti,Pb, Bi, and Ca as optional elements that may be included, and Fe andinevitable impurities, wherein the required elements and the optionalelements are in amounts by weight of: C within a range of 0.25 to 0.32%,Si within a range of 0.01 to 0.15%, Mn within a range of 0.55 to 0.90%,Cu within a range of 0.10 to 0.60%, Ni within a range of 0.05 to 0.30%,Cr within a range of 0.10 to 0.20%, Mo within a range of 0.05% or less,N within a range of 0.020% or less, s-Al within a range of 0.020% orless, and Ti within a range of 0.020% or less, at least one of theoptional elements Pb, Bi, and Ca to improve machinability, and Fe andthe inevitable impurities in a remaining portion, and, satisfyingconditions as follows: 0.430≦C_(eq)≦0.530, whereC_(eq)=C+0.07×Si+0.16×Mn+0.19×Cu+0.17×Ni+0.2×Cr+0.4×Mo, and0.380≦C_(req)≦0.470, whereC_(req)=0.55×C+0.06×Si+0.18×Mn+0.09×Cu+0.47×Mo+1.0×Cr; a Ni to Cu weightratio is 0.50≦Ni/Cu≦1.00; and amounts by weight of P and S, which arethe inevitable impurities, are suppressed to 0.03% or less and 0.15% orless, respectively, wherein the nitrocarburized crankshaft memberincludes a thickness of a surface compound layer of the nitrocarburizedhard layer of 10 to 35 μm that is formed during establishment of adiffusion depth of a nitrogen diffusion zone below the surface compoundlayer of 700 μm or greater.
 2. The nitrocarburized crankshaft memberaccording to claim 1, wherein an HV ratio of a maximum hardness of thenitrogen diffusion zone, at a depth of at least 50 μm or greater fromsaid surface compound layer, to a core hardness is 1.65 or greater. 3.The nitrocarburized crankshaft member according to claim 2, wherein themaximum hardness of the nitrogen diffusion zone is within the range ofHV 280 to
 330. 4. The nitrocarburized crankshaft member according toclaim 2, wherein the core hardness is within the range of HV 160 to 190.5. The nitrocarburized crankshaft member according to claim 4, whereinthe maximum hardness of the nitrogen diffusion zone is within the rangeof HV 280 to 330.